If you buy it in the store, they will activate it right there. That's what the Apple store employees told me today. Which I am really happy about. Get to try out the youtube capabilities immediately. 🙂
Appel chat told me a few days ago that it won't work like the first gen iPhone. You already chose the plan options online so all you have to do is turn it on and plug it in to iTunes... and it supposed to just.... activate. Thats the Apple magic... lol